Circuits, external to a magnetron, have been utilized for altering, or "pulling", the frequency of the electromagnetic radiation produced by the magnetron. One such circuit is shown in the U.S. Pat. No. 3,714,592 which issued in the name of Jory on Jan. 30, 1973. Therein, the output port of a magnetron is coupled via a circulator to a load. A variable phase shifter is coupled between the circulator and the load for varying the impedance reflected back to the magnetron and thereby pulling the frequency thereof.
A problem arises in that phase shifting circuitry in series with a load must necessarily operate with power levels commensurate with the magnitude of power transmitted to the load. In typical situations wherein magnetrons are utilized in communication networks, such a power level precludes the use of relatively small size phase shifters which are capable of rapidly shifting phase, the larger, high powered, phase shifters having a slower response time.